1. Field of the Invention
This invention relates to turbocharger control and more particularly relates to preventing an underspeed event of a turbocharger.
2. Description of the Related Art
Turbochargers are a common feature on combustion engines to enhance performance. Continuing demand for power density and torque combined with emissions regulations have driven innovation in turbocharger hardware and turbocharger control systems. Hardware related to turbocharger control, such as wastegates, relief valves, bypass valves, EGR valves, variable geometry turbines, and adjustable vanes have been known in the art for some time. Furthermore, some contemporary engines have multiple turbochargers to manage competing design criteria. For example, many engines include a small turbocharger for better response at low engine loads. Smaller turbochargers pose additional control challenges as the lower mass of the turbines means that they are more responsive to changes in pressure and mass flow, and thereby more affected by such changes which can cause speed overshoots and undershoots.
Turbo shafts that mechanically couple the turbine to a compressor in the turbocharger may also be at risk for damage if an underspeed event occurs. The thrust load that a turbo shaft may support is related to the lubricating oil film strength on the turbo shaft. Generally, a turbo shaft rotating at a higher speed provides a stronger oil film and therefore supports a higher thrust load than a turbo shaft rotating at a lower speed.
Certain transient events, such as a heavy truck accelerating from a standstill, can cause a high thrust load on the turbocharger while the turbocharger speed is low. Prior to acceleration, the turbocharger is rotating at a low speed and the corresponding turbo shaft is rotating at a low speed, i.e. the turbo shaft has a weak oil film and low thrust load capacity. Initially, the thrust load on the turbocharger shaft is low as the truck is immobile, providing a suitable operational environment for the turbocharger. However, a hard acceleration can cause a large pressure differential across the turbocharger before the mass flow through the turbocharger accelerates the turbocharger to a high speed. Engine braking, engine manipulations to generate temperature for regenerating an aftertreatment device, and other transient events can similarly cause high pressure differentials across the turbocharger while the turbocharger is rotating at a relatively slow speed. The recent prevalence of small, highly responsive turbochargers exacerbates transient problems as the turbocharger is capable of more rapid deceleration while still experiencing a thrust load.